In the laying of sewer lines a laser beam transmitter is positioned in the ditch, and its beam, directed toward the target, serves as a guide for the alignment of the pipes. This beam must be properly aligned to give the desired slope as well as the direction. With the usual instruments the slope is adjusted at the laser beam transmitter, while the direction is determined by a transit or other sighting mechanism set up over the ditch, usually trained on a surveyor's rod or other object set up over the target point. The laser beam transmitter then must be aligned parallel to the sighting mechanism in the horizontal plane.
One well-known arrangement for this purpose connects the laser unit set up below with a telescopic sight set up above by means of a vertical rod. When the two instruments are fastened to the rod parallel, the sight can be aligned with the target, with the laser unit remaining parallel. The disadvantage of this arrangement is, however, the rod connecting the two instruments, which gets in the way of the construction work and greatly increases the amount of apparatus needed, the likelihood of disturbance, servicing required, etc.
Another system of this general type is familiar from U.S. Pat. No. 3,667,849. The arrangement involves a laser beam transmitter positioned over the construction ditch, which serves to take bearings on the distant target. Two beams are separated out from the original path of the transmitter and are then reflected down in the plane of the instrument's principal axis. An azimuth transfer device set up below in the ditch receives these two beams by means of windows lying in a plane parallel to its principal axis. Electronic sensers positioned under the windows serve to monitor the precise alignment. One of the beams received has its direction altered by 90.degree. and is used as a directional beam in the construction ditch. This arrangement transmits an azimuth optically by means of precisely aligned laser beams, making a high degree of exactness possible. The troublesome connecting rod of the arrangement precisely discussed is no longer necessary.
This arrangement does, however, have the disadvantage that the sensing device only registers a response when the two instruments are already so precisely parallel and vertically aligned that the two parallel beams pass through the windows. The work crew must align the two units by hand until the two beams pass through the windows, a time consuming task when the two instruments are fairly far apart. If the instrument on top is set up at an angle to the horizontal--if, for example, the target is not in a horizontal plane with the instrument--the alignment of the two units is made extraordinarily difficult.
A further disadvantage is the fact that the sensing device combines optical and electronic elements. Electronic equipment is much too fragile for constant use on a construction site.
The object of this invention is, therefore, to devise a system of the type first mentioned, which will make possible quick and precise alignment of the two instruments without complicated electronic equipment and even when the instruments are set up under unfavorable circumstances.
The invention attains this object by providing (a) fixed optical means on the laser beam transmitter for producing a fan-shaped beam and (b) an eyepiece on the receiving device for the simultaneous observation of the two windows via appropriate optical means. The fan-shaped beam products at its point of impact a clearly visible line. If this beam falls on the surface of the instrument that is to be positioned, a line will be visible on that surface, which will make approximate alignment of the unit a simple matter. Since the beam produces a clearly visible line several meters long at the point of impact, given the usual distance of several meters between the two instruments, the two need no longer be set up in a vertical line. They can be shifted with respect to each other or set up with a relative angle of inclination. Thus the work of alignment is extraordinarily simplified and accelerated. The receiving device is much sturdier than the familiar electronic sensing device, since it involves only fixed optical elements.
The receiving device in the invention has a further advantage that both windows may be observed simultaneously via the eyepiece. That is, the operator sees the fan-shaped beam in both windows simultaneously when looking through the eyepiece, and he can perform precise alignments by utilizing the eye's ability to discriminate slight movements of the two visible lines relative to each other. Alignment according to the invention is at the very least the equal of alignment by electronic means.
Producing a fan-shaped beam from a laser beam is, of course, familiar from DT-OS 2 403 239. The arrangement described there produces such a beam by means of a rotating mirror driven by an electric motor. This arrangement represents the most advanced technology up to now in the area of construction lasers. Still, the arrangement with rotating mirror is only advantageous when angles up to 360.degree. are to be utilized. In the case of the present arrangement this capacity is not necessary. Besides that, an arrangement with rotating mirrors would be too expensive for this purpose.
The system of this invention has the further advantage that the fixed optical means used to produce the fan-shaped beam consists of a cylindrical lens located in the path of the laser beam. Compared with the familiar rotating means for producing a fan-shaped beam, a cylindrical lens is very sturdy and extraordinarily simple in construction.
The system of the invention has the further advantage that the contrivance for projecting the beam involves a one-dimensional phase plate in the path of the laser beam, with its edge parallel to the principal axis of the instrument. Phase plates serve to produce a phase gap along one edge, where the phase plate has a thickness of the order of magnitude .lambda./2 of the length of light wave. With a coherent beam like a laser beam, a very sharp line of intensity zero is produced, which is still extraordinarily sharp at a considerable distance. The use of a phase plate with laser beams is described in detail in DT-OS 1 673 846 (now U.S. Pat. No. 3,617,135). Using a phase plate has the advantage that the bending of the beam at the edge makes a relatively long line along the zero line. Even without an additional cylindrical lens, a phase plate alone is suitable for producing an appropriate fan-shaped beam with a precise zero line. Use of a phase plate and a cylindrical lens in succession has the advantage of giving a longer zero line. In the present case the precision of alignment is increased by the use of a phase plate, since the zero line is very much sharper than the usually diffuse and relatively wide laser beam.
The system of the invention has the further advantage that the set of reflectors in the receiving device employs precisely congruent angles. The use of such a system allows the observer to tell exactly how much he must adjust the unit in order to bring it into precise alignment with the transmitter.
The system of the invention has the further advantage that the windows which allow beams to enter the receiving device are provided with ground glass screens. The clear picture on such a screen permits the beams to be clearly observed, even when skewed or off center. The ground glass screens can be advantageously marked with reference lines parallel to the principal axis of the device, which allow one to observe not only whether the receiving device is skewed but also whether the principal axis of the receiving unit coincides with the transmitted beam.
The system of the invention has the further advantage that the set of optical elements includes a five-sided prism and a 90.degree. prism, positioned normal to the principal axis of the device, each of which, with the aid of a reflector, permits the operator to view one of the windows through the eyepiece. A five-sided prism has the property that the angle between the original beam and the refracted beam is always 90.degree.. If the direction of the original beam is altered, the refracted beam is altered by the same angle and in the same direction. With a 90.degree. prism, if the direction of the original beam is altered by a certain angle, the reflected beam is altered in the opposite direction but still by the same amount. So if the images on the ground glass screens move in opposed directions when the receiving unit is set at an angle to the fan-shaped beam, the images seen in the eyepiece through one or the other of the prisms will move apart also. Movement of the receiving unit parallel to the fan-shaped beam will show up as a simultaneous movement in the same direction of both images seen through the eyepiece. Thus, aligning the device is a simple matter, even for the untrained observer, since the two images will move relative to each other, at least when the alignment is skewed.
The system of the invention has the further advantage that the contrivance for projecting the laser beam is a part of the laser beam transmitter and alters the direction of its beam by 90.degree., before changing it into a fan-shape, by means of an adjustable reflector which is moveable into and out of the original path of the beam. In this way only one laser generator is needed, the one that is in any case a part of the transmitter. The reflector is only needed during alignment and can then be moved out of the path of the beam, so that the beam is directed forward as usual. An additional advantage here is the fact that the receiving unit is attached to a sighting mechanism, for example a telescopic sight, so that the alignment of the latter and the parallel positioning of the two units can be monitored in one place.
The system of the invention has the alternative advantage of mounting a receiving device on the laser beam transmitter, which automatically adjusts the axis of the transmitter to be parallel to the fan-shaped beam. In this arrangement only the telescopic sight or other sighting mechanism needs to be monitored, since the laser beam transmitter, which may in some cases be somewhat difficult to get to, automatically aligns itself. Means for automatic alignment are more easily built into the transmitter, since this unit will have various mechanisms for adjustment anyway.
The system of the invention has the final advantage that the laser beam transmitter may have an automatic levelling mechanism. This device, which will automatically keep the unit horizontal even when the ground under it is being moved and thus maintain the proper slope of the beam, allows the operator to concentrate solely on the alignment of the telescopic sight and the parallel alignment of the two units. It is difficult construction situation the work will be greatly simplified if one of the parameters of alignment does not need to be attended to, so that the surveying work will be done more accurately.